Electrosurgical generators are used to deliver therapeutic electrical energy to surgical instruments. These instruments are used for example, for cutting, coagulation, tissue welding, ablation, and dissection. Both monopolar and bipolar generators typically supply electrosurgical energy in the radio frequency (RF) range to such instruments. Usually such generators include controls that regulate the voltage and/or current so that a select power level is approximately delivered and a maximum power level is not exceeded.
When such RF generators are used, the primary control is the experience of the surgeon who responds to what is observed to be happening to the tissue as it is treated with the RF energy. Often, particularly for endoscopic procedures, surgeons cannot readily see what is happening to the tissue. Also, the change in tissue properties due to the RF energy may occur so quickly so as not to afford time for the surgeon to react soon enough to turn off electrical energy to the instrument. As a result, some problems which may occur include tissue charring, sticking of tissue to the electrodes of the surgical instrument, and over or under treatment of tissue.
It has been recognized that the tissue impedance changes as RF energy is applied to the tissue. Attempts have been made to control the power delivered to the tissue as the tissue impedance changes. For example, current has been controlled based on the change in the voltage or the power delivered by the generator to tissue. The differential quotient of tissue impedance as RF power is applied to the tissue has been used to determine an initial power level and to switch off RF power when the differential quotient of impedance reaches a preset value.
Notwithstanding these control arrangements, there is a continuing need for improvement in the control of electrosurgical energy deliverance to the tissue and/or determination of when tissue treatment has reached an optimal level.
In particular there is a need to provide a device and method for determining the end point of coagulation for a variety or a range of tissue impedance, due to, e.g., varying tissue composition, tissue types and treated tissue areas and/or volumes.